Arsenic is a well-documented human carcinogen. The present study explored the role of the onco-miR, miR-21 and its target protein, programmed cell death 4 (PDCD4) in arsenic induced malignant cell transformation and tumorigenesis. Our results showed that treatment of human bronchial epithelial (BEAS-2B) cells with arsenic induces ROS through p47phox, one of the NOX subunits that is the key source of arsenic-induced ROS. Arsenic exposure induced an upregulation of miR-21 expression associated with inhibition of PDCD4, and caused malignant cell transformation and tumorigenesis of BEAS-2B cells. Indispensably, STAT3 transcriptional activation by IL-6 is crucial for the arsenic induced miR-21 increase. Upregulated miR-21 levels and suppressed PDCD4 expression was also observed in xenograft tumors generated with chronic arsenic exposed BEAS-2B cells. Stable shut down of miR-21, p47phox or STAT3 and overexpression of PDCD4 or catalase in BEAS-2B cells markedly inhibited the arsenic induced malignant transformation and tumorigenesis. Similarly, silencing of miR-21 or STAT3 and forced expression of PDCD4 in arsenic transformed cells (AsT) also inhibited cell proliferation and tumorigenesis. Furthermore, arsenic suppressed the downstream protein E-cadherin expression and induced &beta;-catenin/TCF-dependent transcription of uPAR and c-Myc. These results indicate that the ROS-STAT3-miR-21-PDCD4 signaling axis plays an important role in arsenic -induced carcinogenesis.

Mentions:
Previous studies reported that suppression of PDCD4 elicits an inhibition of E-cadherin expression, which is associated with an increase in β- catenin activation and TCF4 expression31. To confirm this in our model system, BEAS-2B cells were exposed to various doses of arsenic for a short (24 h) or extended (6 mo) time period. Results show that both acute (Fig. 9A) and chronic (Fig. 9B) arsenic treatment of BEAS-2B cells down-regulated E-cadherin protein expression with an associated up-regulation of active β-catenin (nuclear translocated form) and TCF4, whereas the level of total β-catenin remained unchanged. Immunofluorescence staining confirmed that chronic arsenic exposure decreased E-cadherin expression (Fig. 9C) and increased β- catenin (Fig. 9D) and TCF4 accumulation in the nucleus (Fig. 9E). These data clearly show that arsenic exposure suppresses E-cadherin and increases the accumulation of β- catenin and TCF4 in the nucleus during malignant transformation.

Mentions:
Previous studies reported that suppression of PDCD4 elicits an inhibition of E-cadherin expression, which is associated with an increase in β- catenin activation and TCF4 expression31. To confirm this in our model system, BEAS-2B cells were exposed to various doses of arsenic for a short (24 h) or extended (6 mo) time period. Results show that both acute (Fig. 9A) and chronic (Fig. 9B) arsenic treatment of BEAS-2B cells down-regulated E-cadherin protein expression with an associated up-regulation of active β-catenin (nuclear translocated form) and TCF4, whereas the level of total β-catenin remained unchanged. Immunofluorescence staining confirmed that chronic arsenic exposure decreased E-cadherin expression (Fig. 9C) and increased β- catenin (Fig. 9D) and TCF4 accumulation in the nucleus (Fig. 9E). These data clearly show that arsenic exposure suppresses E-cadherin and increases the accumulation of β- catenin and TCF4 in the nucleus during malignant transformation.

Arsenic is a well-documented human carcinogen. The present study explored the role of the onco-miR, miR-21 and its target protein, programmed cell death 4 (PDCD4) in arsenic induced malignant cell transformation and tumorigenesis. Our results showed that treatment of human bronchial epithelial (BEAS-2B) cells with arsenic induces ROS through p47phox, one of the NOX subunits that is the key source of arsenic-induced ROS. Arsenic exposure induced an upregulation of miR-21 expression associated with inhibition of PDCD4, and caused malignant cell transformation and tumorigenesis of BEAS-2B cells. Indispensably, STAT3 transcriptional activation by IL-6 is crucial for the arsenic induced miR-21 increase. Upregulated miR-21 levels and suppressed PDCD4 expression was also observed in xenograft tumors generated with chronic arsenic exposed BEAS-2B cells. Stable shut down of miR-21, p47phox or STAT3 and overexpression of PDCD4 or catalase in BEAS-2B cells markedly inhibited the arsenic induced malignant transformation and tumorigenesis. Similarly, silencing of miR-21 or STAT3 and forced expression of PDCD4 in arsenic transformed cells (AsT) also inhibited cell proliferation and tumorigenesis. Furthermore, arsenic suppressed the downstream protein E-cadherin expression and induced &beta;-catenin/TCF-dependent transcription of uPAR and c-Myc. These results indicate that the ROS-STAT3-miR-21-PDCD4 signaling axis plays an important role in arsenic -induced carcinogenesis.